Preparation of silicon tetrachloride



1964 w. R. BARNES ETAL 3,122,492

PREPARATION OF SILICON TETRACHLORIDE Filed Dec. 23, 1960 United States Patent PREZARATION OF S-iIJICQN TETRACHLSR-IDE William Richard Barnes, Knutsiord, and Hamid Garton Emblem, Grappenhall, England, assiguors to Philadelphia Quartz Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Dec. 23, 1960, Ser. No. 78,141 Claims priority, application Great Britain Jan. 1, 1960 Claims. (Cl.-204-1S7) This invention relates to the production of silicon tetrachloride.

According to the invention, there is provided a process for the preparation of silicon tetrachloride in which chlorine gas is irradiated with ultra violet light and reacted with a metal alloy of silicon or a metal silicide to produce silicon tetrachloride. 1

Embodiments of the invention will now be described with reference to the accompanying diagrammatic drawing in which FIGURE I shows a perspective view of part of an apparatus for preparing silicon tetrachloride in accordance with the invention, and

FIGURE II shows a similar view of the apparatus shown in FIGURE 1.

In FIGURE I, reference 1 indicates a horizontal silica tube at one end of which there is provided an inlet tube 2 and at the other end an outlet tube 3. The tube 1 is surrounded by a metallic reflector structure constituted by two similar elongated cylindrical reflectors 4 and 5 each having a transverse cross-section in the form of an arc of an ellipse; the reflectors are arranged relative to one another in such manner that they have a common focal line. In the drawing the reflectors 4 and 5 are shown partly broken away, the continuation of the parts broken awaybeing shown by broken lines. The tube another form of p 1 is arranged within the reflector structure with its axis substantially coincident with the common focal line of the elliptical reflectors 4 and 5. At the other focal lines of the reflectors 4 and 5 are arranged tubular ultra-violet lamps 6 and 7, respectively. The reflector structure 4, 5 is therefore adapted to focus ultra-violet light emitted by the lamps 6 and 7 onto the tube 1, which tube is transparent to the ultra-violet light. Within the tube 1 is a mass of finely divided terrosilicon alloy (not shown);

suitable alloys are those containing from 10 to 70% or even more of iron, the balance being silicon save for traces of impurities which are normally present in ferrosilicon alloys such as, for example, carbon, aluminium and calcium. The alloy' should be finely divided, for example sufficiently finely divided to pass a 60 mesh British standard sieve, which sieve has apertures of width 0.251

Using the apparatus described above the process of the invention was carried out as follows. Chlorine gas was introduced through the inlettube 2 and irradiated by ultra-violet radiation irom the lamps 6 and 7. The silicon tetrachloride produced as the gas passedover and reacted with the ferrosilicon alloy flowed through the outlet tube 3 and was condensed in known manner (not shown).

By carrying out the process as described and illustrated it was found unnecessary to supply tothe reaction tube 1 any form of heat other than that produced by the ultraviolet lamps 6 and 7. Employing for each of the lamps 6 and 7 a 400 watt mercury vapour lamp, the reaction proceeded satisfactorily and the temperature of the ferrosilicon mass was estimated to be about 75 to FIGURE II shows apparatus similar to that of FIG- URE I in which the reflectors 4 and 5 are combined into one unit and, instead of being elliptical, are parabolic.

The ultraviolet lamps 6 and 7 are positioned at the foci of the parabolic reflectors each of which reflects the radiation from the correspondinglamp as a parallel beam onto the reaction 'tube 1. This construction enables a reaction tube 1 of greater diameter to be employed. The tube 1 may be made of materials other than silica, such as a borosilicate glass,-provided it is transparent or substantially transparent to the ultra-violet radiation employed. j

The effect, on the heterogeneous gas/solid reaction between the chlorine and the ferrosilicon alloy, of irradiating the chlorine with ultra-violet light was considerable and unexpected. When irradiation was omitted it was found necessary to heat the ferrosilicon alloy' to at least about 300 C. to obtain a satisfactory reaction rate.

Even at this temperature, the reaction did not begin for some considerable time, the induction period often being an hour or more, which was a considerable inconvenience as the chlorine passing over during this induction period was wasted. However, by irradiating the chlorine with ultra-violet light in accordance with the invention the reaction occurred at a very much lower temperature and I a substantial reduction in the induction period was obtained. A further advantage of the/process of the in-.

vention which was obtained when ferrosilicon alloy was employed was that there was a considerable reduction in theamount of by-product ferric chloride sublimed over with the silicon tetrachloride because of the lower operating temperature; in most cases the silicon tetrachloride obtained was virtually free of ferric chloride.

Instead of using an alloy of iron and silicon, an alloy of silicon and copper may be used. Furthermore, the

Experiment 1 Using the apparatus shown in FIGURE 1, dry chlorine was passed over finely-divided fer-rosilicon alloy containing iron in silica tube 1 of diameter about 4 ems. The alloy was sufiiciently finely-divided to all pass a 60 mesh British standard sieve; The chlorine as it passed over the ferrosilicon alloy was irradiated with ultra-violet light from two 400 watt mercury vapour lamps 6 and '7. The induction period was about 10 to 15 minutes and the reaction temperature of the mass of alloy was about -80 C. The production rate of silicon tetrachloride was 13.4 g. per hour. The silicon tetrachloride product obtained was yellow in colour and was substantially free of iron chloride. To remove the dissolved chlorine it was necessary merely to heat the product under reflux when a clear liquid was obtained.

Experiment 2 To show the eifect of the irradiation, a similar experiment was performed without irradiating the chlorine.

The ferrosilicon was heated in the darkto a temperature of 280 C. to start the reaction.

induction period was about 3 /2 hours. The rate of production of silicon tetrachloride, once begun, was the Patented Feb. 25, 1964 In this case, the p pure product it was necessary to distill oil the silicon tetrachloride from the crude mixture, dissolved chlorine being also removed in this operation.

What is claimed is:

1. A process for preparing silicon tetrachloride in which chlorine gas irradiated with ultra-violet light is reacted with a finely divided silicon-containing material selected from the group consisting of metal alloys of silicon and metal silicides at a temperature not greater than 300 C. to produce silicon tetrachloride.

2. A process as claimed in claim 1, in which the heat required to heat the reactants to a temperature at which reaction occurs is provided by' the source of ultra-violet radiation.

3. A process as claimed in claim 1, in which after the reaction between chlorine and silicon alloy has started, the irradiation is stopped and the reactants heated to a suitable higher temperature at which the reaction can continue in the absence of the radiation.

4. A process as claimed in claim 1 in which the siliconcontaining material employed is ferrosilicon.

5. A process as claimed in claim *1, in which the ultraviloct radiation is provided by a mercury vapour lamp.

References (Iited in the file of this patent UNITED STATES PATENTS 2,617,761 Sheer et al Nov. 11, 1952 2,882,211 Autrey 2. Apr. 14, 1959 

1. A PROCESS FOR PREPARING SILICON TETRACHLORIDE IN WHICH CHLORINE GAS IRRADIATED WITH ULTRA-VIOLET LIGHT IS REACTED WITH A FINELY DIVIDED SILICON-CONTAINING MATERIAL SELECTED FROM THE GROUP CONSISTING OF METAL ALLOYS OF SILICON AND METAL SILICIDES AT A TEMPERATURE NOT GREATER THAN 300*C. TO PRODUCE SILICON TETRACHLORIDE. 